In recent years, influenced by a trend to reduce the reliance on fossil fuels, research of electric automobiles is advancing, and electric automobiles have in part already been put into practical use. The electric motor, which is the power source of an electric automobile, differs from an internal combustion engine that runs by direct combustion of fossil fuels in that the torque and rotational speed characteristics of an output shaft, such as typically generating maximum torque at start up, are preferable for an automobile, so it is not absolutely necessary to have a transmission that is required by a typical automobile having an internal combustion engine as the driving source. However, even in the case of an electric automobile, by providing a transmission, it is possible to improve the accelerating performance and high-speed performance. More specifically, by providing a transmission, the relationship between the traveling speed and acceleration of an automobile is close to that of a gasoline engine automobile in which a transmission is provided in the power train, and smooth driving is possible. This will be explained with reference to FIG. 19.
For example, when a power transmission apparatus having a large reduction ratio is provided between the output shaft of an electric motor, which is the driving source of an electric automobile, and the input section of a differential gear, which is connected to the drive wheels, the relationship between the acceleration (G) and the traveling speed (km/h) of an electric automobile is as illustrated by left half of the solid line “a” being continuous with the chain line “b” in FIG. 19. In other words, the acceleration performance at low speed is excellent, however, high-speed travel becomes impossible. On the other hand, when a power transmission apparatus having a small reduction ratio is provided between the output shaft and the input section, the relationship between the acceleration and the traveling speed is as illustrated by the chain line “c” being continuous with the right half of the solid line “a” in FIG. 19. In other words, high-speed travel becomes possible, however the acceleration performance at low speed is impaired. However, by providing a transmission between the output shaft and the input section and changing the reduction ratio of that transmission according to the vehicle speed, it is possible to obtain a characteristic such as the left half and the right half of the solid line “a” being continuous. This characteristic is nearly equivalent to a typical gasoline engine automobile having the same amount of output power as illustrated by the dashed line “d” in FIG. 19, and in regards to the acceleration performance and high-speed performance, by providing a transmission, it is known that even in an electric automobile it is possible to obtain performance equivalent to that of a gasoline engine automobile.
FIG. 20 illustrates an example of conventional construction of a drive apparatus for an electric automobile in which a transmission is provided between the output shaft of the electric motor and the input section of a differential gear that is connected to the drive wheels, as disclosed in JP 2006-022879 (A). This drive apparatus for an electric automobile is constructed so as to rotate and drive a pair of left and right drive wheels by transmitting the rotation of the output shaft of an electric motor 1 to a rotation transmission apparatus 3 by way of a transmission 2. In the transmission 2, there is a pair of gear transmission mechanisms 6a, 6b that are located between a drive-side rotating shaft 4 that is concentric with the output shaft of the electric motor 1 and a driven-side rotating shaft 5, and these gear transmission mechanisms 6a, 6b have different reduction ratios. By switching a pair of clutch mechanisms 7a, 7b, only one of the gear transmission mechanisms 6a, 6b is placed in a state capable of transmitting power, making it possible to switch the reduction ratio between the drive-side rotating shaft 4 and the driven-side rotating shaft 5 between two steps: large and small.
More specifically, one clutch mechanisms 7a of these clutch mechanisms 7a, 7b can be controlled by an actuator, while the other clutch mechanism 7b is an overrunning clutch that becomes disengaged when the rotational speed becomes a fixed value or greater. When the one clutch mechanism 7a is engaged, the other clutch mechanism 7b becomes disengaged, and the rotation torque of the drive-side rotating shaft 4 is transmitted to the driven-side rotating shaft 5 by way of the gear transmission mechanism 6a of the gear transmission mechanisms 6a, 6b that has a small reduction ratio. When the one clutch mechanism 7a is disengaged, the other clutch mechanism 7b becomes engaged, and the rotation torque of the drive-side rotating shaft 4 is transmitted to the driven-side rotating shaft 5 by way of the other gear transmission mechanism 6b having a large reduction ratio. The rotation of the driven-side rotating shaft 5 is transmitted to the input section of the differential gear 8 by a rotation transmission apparatus 3, and as a result, the output shafts 9a, 9b that support the pair of left and right drive wheels are rotated and driven.
In the case of this kind of conventional construction, a pair of gear transmission mechanisms 6a, 6b are provided between the drive-side rotating shaft 4 and driven-side rotating shaft 5 so as to be separated in the radial direction and parallel with each other. Moreover, the gear transmission mechanism 6a (6b) transmits power by engagement of a gear 10a (10b) that is provided in the middle section in the axial direction of the drive-side rotating shaft 4 and a gear 10c (10d) that is provided in the middle section in the axial direction of the driven-side rotating shaft 5. Therefore, the gears 10a, 10c (10b, 10d), which engage with each other, must have sufficient strength and durability so as to be able to transmit all of the power that is outputted from the electric motor 1. Therefore, there is a problem in that the drive apparatus for an electric automobile in which gear transmission mechanisms 6a, 6b are assembled becomes large and the weight increases.
The one clutch mechanism 7a requires an actuator for switching the engaged and disengaged (engagement) state. In other words, the one clutch mechanism 7a is provided in the middle section in the axial direction of the drive-side rotating shaft 4, and with the drive-side rotating shaft 4 in a rotating state, must switch between the engaged and disengaged state of the gear of one gear transmission mechanism 6a of the pair of the gear transmission mechanism 6a, 6b (between state in which the drive-side rotating shaft 4 and gear 10a rotate in synchronization, and state in which the gear 10a idles with respect to the drive-side rotating shaft 4). Therefore, an electromagnetic clutch is used as the one clutch mechanism 7a, so there is a possibility that the construction of the drive apparatus for an electric automobile will become complex. Moreover, in this case, it also becomes difficult to maintain the torque transmission capacity.
In order to lengthen the traveling distance per one charge in order to improve the convenience of an electric automobile, making the drive apparatus for an electric automobile more compact and lightweight, and reducing the amount of power consumed per traveling distance is essential. As technology for making the drive apparatus for an electric automobile more compact, JP 2010-090947 (A) and JP 2010-223298 (A) disclose technology in which rotating shafts that are connected to a transmission and that have different reduction ratios from each other are provided on both the inner-diameter side and outer-diameter side of the tube shaped output shaft of an electric motor so as to be concentric with the output shaft, where one rotating shaft of the inner-diameter-side rotating shaft and outer-diameter-side rotating shaft is rotated and driven by a pair of clutches. However, in the construction disclosed in these patent literatures as well, transmission of power between a pair of rotating shafts is performed by engagement of a pair of gears. The clutch must have an actuator that, with a rotating shaft rotating, switches the gear that is provided in the middle section in the axial direction of the rotating shaft between a state of rotating in synchronization with the rotating shaft, and a state of idling with respect to the rotating shaft, and from the aspect of simplifying the construction of a drive apparatus for an electric automobile, there is room for improvement.